Ion source filament and method

ABSTRACT

Ion source filaments, as well as methods and apparatus associated with the same are provided. The source filaments have a design that includes a relatively small surface area from which electrons are emitted (i.e., active portion) as compared to certain conventional source filaments. Suitable designs include filaments that have a V-shape or U-shape active portion, rather than a coiled active portion as in certain conventional source filaments. The source filaments of the present invention can increase the efficiency of ion generation and, in particular, the generation of multiply charged ionic species. The increased ion generation efficiency may enable formation of ion beams having relatively high beam currents suitable for implantation.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. provisional patent application serial No. 60/281,070, filed Apr. 3, 2001, entitled “Method to Enhance Helium Ion Production In An Ion Source Apparatus”, and U.S. provisional patent application serial No. 60/281,069, filed Apr. 3, 2001, entitled “Multi-Charge Filament”, the disclosures of which are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

[0002] The invention relates generally to ion implantation and, more particularly, to an ion source filament, as well as methods and apparatus associated with the same.

BACKGROUND OF THE INVENTION

[0003] Ion implantation is a conventional technique for introducing dopants into semiconductor materials. An arc discharge may be generated within an arc chamber of an ion source to ionize a desired dopant gas. The ions may be extracted from the source to form an ion beam of selected energy which can be directed at the surface of a semiconductor wafer. The ions in the beam penetrate into the semiconductor wafer to form an implanted region.

[0004] Some types of ion sources include an electrically resistive filament located within the arc chamber. To generate the arc discharge, current is passed through the filament while a voltage is applied between the filament and a positive electrode. Suitable filaments can be made of tungsten or tantalum. One conventional filament design, known as a Bernas-type filament, includes a coil at its tip. Other filament types and designs are also known.

[0005] It is desirable in certain ion implantation processes to increase the efficiency of the generation of ionic species such as multiply-charged ionic species. Increasing ionization efficiency, for example, can enable formation of ion beams with increased beam current. Techniques for enhancing ionization efficiency include increasing the current through the filament or the applied voltage so as to provide more arc power. However, such techniques typically result in reducing the operational life of the filament which can sacrifice performance of an ion implanter and can increase expense.

SUMMARY OF THE INVENTION

[0006] The invention is directed to ion source filaments, as well as methods and apparatus associated with the same.

[0007] In one aspect, the invention provides an ion source. The ion source includes an arc chamber, and a filament having at least a portion that is located in the arc chamber. The filament includes a pair of arm members joined by a non-coiled tip portion, wherein the tip portion defines a V-shape or a U-shape.

[0008] In another aspect, the invention provides a method of using a filament in an ion source. The method includes using a first filament that includes an active portion having a first active surface area at first source operating conditions to generate source gas ions at a first efficiency. The method includes replacing the first source filament with a second source filament. The second source filament includes an active portion having a second active surface area less than the first active surface area. The method further includes using the second source filament at the first source operating conditions to generate the source gas ions at a second efficiency greater than the first efficiency.

[0009] Other aspects, features and advantages will become apparent from the following detailed description and drawings when considered in conjunction with the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 schematically illustrates an ion implantation system that may be used in connection with embodiments of the invention.

[0011]FIG. 2a schematically illustrates an ion source that includes a conventional filament.

[0012]FIG. 2b schematically illustrates an ion source that includes a filament according to one embodiment of the invention.

[0013]FIG. 3 is a side view of a filament according to one embodiment of the invention.

[0014]FIG. 4 is a top view of the filament of FIG. 3.

[0015]FIG. 5 is a graph comparing P⁺⁺ beam currents at various arc voltages obtained using a conventional filament and a filament of the invention as described in Example 1.

[0016]FIG. 6 is a graph comparing P⁺⁺⁺ beam currents at various arc voltages obtained using a conventional filament and a filament of the invention as described in Example 1.

[0017]FIG. 7 is a graph comparing P⁺⁺ beam currents at various arc currents obtained using a conventional filament and a filament of the invention as described in Example 2.

DETAILED DESCRIPTION

[0018] The invention provides ion source filaments, as well as methods and apparatus associated with the same. The source filaments have a design that includes a relatively small surface area from which electrons are emitted (i.e., active portion) as compared to certain conventional source filaments. Suitable designs include filaments that have a V-shape or U-shape active portion, rather than a coiled active portion as in certain conventional source filaments. As described further below, the source filaments of the present invention can increase the efficiency of ion generation and, in particular, the generation of multiply charged ionic species. The increased ion generation efficiency may enable formation of ion beams having relatively high beam currents suitable for implantation.

[0019] A schematic block diagram of a typical ion implantation system 10 is shown in FIG. 1. An ion source 12 of the system includes a source gas supply 14 connected to an arc chamber 16. As described further below, an arc discharge is generated in the arc chamber by passing a current through a filament and applying a voltage to a filament. The arc discharge includes ionized source gas molecules. The ions may be extracted from the ion source to form an ion beam 18 which is directed along a beam path toward a target, such as a semiconductor wafer 20.

[0020] Ion beam 18 is deflected and focused by a mass analyzing magnet 22. Downstream of the mass analyzing magnet 22, the ion beam may be focused in the plane of a mass resolving slit assembly 26. The ion beam 18 is accelerated to a desired energy by an accelerator 28 and impinges on wafer 20 located within an end station 29. The entire region between ion source 12 and wafer 20 is evacuated during ion implantation.

[0021] The ion beam 18 may be distributed over the surface of wafer 20 by mechanically scanning the wafer with respect to the beam, by scanning the ion beam with respect to the wafer or by a combination of these techniques. The wafers may be, for example, mounted on a rotating disk during ion implantation. End station 29 may include a system for automatically loading semiconductor wafers into one or more wafer positions for implantation and for removing the wafers from the wafer positions after ion implantation. The ion implantation system may include other components, not shown but known to the skilled person in the art, such as a dose measuring system, an electron flood system, and a tilt angle monitoring system, among others.

[0022]FIG. 2A shows an arc chamber 30 of an ion source that includes a filament 32 a having a conventional design. In this illustrative embodiment, filament 32 a includes a coiled tip portion 34 a. FIG. 2B shows an arc chamber 30 of an ion source that includes a filament 32 b having a design according to the present invention. Filament 32 b includes a non-coiled tip portion 34 b that is V-shaped. Active portions 36 a, 36 b of respective filaments 32 a, 32 b extend into the arc chamber a distance A. As used herein, the term “active portion” refers to the portion of the filament that is located within the arc chamber. Active portions 36 a and 36 b have similar diameters, however the total length of active portion 36 b is shorter than the length of active portion 36 a. Therefore, the surface area of active portion 36 b is less than the surface area of active portion 36 a. As described further below, the smaller active surface area enables the filament of the invention (e.g., 32 b) to generate ions at a greater efficiency than a conventional filament (e.g., 32 a).

[0023] It should also be understood that filament designs of the present invention may also include a smaller active surface area than conventional filaments that have designs that do not include a coiled tip portion.

[0024] During use, gas molecules from supply 14 (FIG. 1) are fed into the chamber through a port 38. Current is passed through filament 32 a (32 b, FIG. 2b), causing active portion 36 a (36 b, FIG. 2B) to heat up and thermionically emit electrons from its surface. A voltage (i.e., arc voltage), for example between about 30 and about 150 volts, is applied between the filament and a positive electrode, such as a chamber wall. The electrons emitted from the filament collide with the gas molecules to generate an arc discharge that includes source gas ions. A magnetic field may also be applied perpendicular to the electric field to increase the electron path within the apparatus and to increase the probability of collisions with gas molecules within the chamber. As described above, the source gas ions may be extracted to form ion beam 18 (FIG. 1).

[0025] It is believed that the smaller surface area of active portion 36 b as compared to active portion 36 a causes the active portion 36 b to be heated to a higher temperature than active portion 36 a at the same operation conditions (i.e., filament current, arc voltage, etc.). The higher temperature results in electrons of higher energy being thermionically emitted from active portion 36 b. The higher electron energies can increase the frequency of collisions that are capable of ionizing gas molecules. Thus, greater ionization efficiencies may be achievable by using filament 32 b as compared to filament 32 a operating at the same conditions.

[0026] Furthermore, it is also believed that because active portion 36 b has a smaller surface area than active portion 36 a, electron emission from active portion 36 b is localized in a smaller region than that from active portion 36 a. The region around active portion 36 b, therefore, includes an increased density of electrons as compared to the region around active portion 36 a. The increased density of electrons enhances the probability that a source gas molecule in that region can be multiply ionized, for example, via highly energetic collisions with one or more electrons. This is also believed to increase the ionization efficiency of filament 32 b as compared to filament 32 a at the same operating conditions and, in particular, with respect to the generation of multiply charged ions.

[0027] The greater ionization efficiencies achievable using filaments of the present invention are generally obtained without sacrificing filament life. This represents an advantage over certain conventional techniques for increasing ionization efficiency, such as techniques that involve increasing the arc power by either increasing the arc current and/or arc voltage, both of which can reduce filament life.

[0028] The dimensions of filaments of the invention depend in part upon the system and process in which they are used. It is generally desirable for the filament to have a similar cross-sectional area and for the filament to extend into the chamber the same distance (e.g., A in FIG. 1) as conventional filament designs. This can increase the compatibility of filaments of the invention with existing ion implantation systems and can facilitate replacing conventional filaments with filaments of the invention. As described above, filaments of the invention may have a reduced active portion length as compared to conventional filaments. In some embodiments, the length of the active portion of filaments of the invention (e.g., 32 b) is between about 50% and about 80% of the length of the active portion of conventional filaments (e.g., 32 a). In some embodiments, the length of the active portion of filaments of the invention is between about 60% and about 70% of the length of the active portion of conventional filaments. For example, a filament of the present invention which has an active portion length of about 1.3 inches can be used to replace a conventional filament that has an active portion length of about 2.0 inches and includes a coiled tip portion.

[0029] However, it should be understood that in some embodiments filaments of the invention may have the same length as conventional filaments. In these embodiments, the smaller active surface area of filaments of the invention may be as a result of a smaller cross-sectional area.

[0030]FIGS. 3 and 4 further illustrate a design of filament 42 according to one embodiment of the invention. As shown, filament 42 includes substantially parallel arm members 44, 46 which are joined by a V-shaped tip portion 48. In other embodiments, tip portion may be U-shaped and/or may define a radius of curvature. In other embodiments, tip portion may have other shapes. It is also possible for arm members to be non-parallel.

[0031] In the embodiment of FIGS. 3 and 4, arm members 44, 46 define a first plane B which intersects a plane C defined by tip portion 48 to form an angle D. This design may facilitate positioning tip portion 48 proximate to the gas inlet in the arc chamber which may be preferred in some cases. Because tip portion 48 is typically the hottest portion of the filament, locating the tip portion near the gas entry port can increase the density of emitted electrons in this area which can enhance ionization efficiency.

[0032] It should also be understood that arm members 44, 46 and tip portion 48 may be in the same plane in some embodiments of the invention.

[0033] Filaments used in connection with the present invention may be made of tungsten, tantalum, or other suitable materials known in the art.

[0034] The filaments of the invention may be used in any suitable ion implantation system. The filaments may enhance ionization efficiency of any type of source gas. However, the filaments may be particularly useful for increasing production of ions from a source gas that has a high ionization potential such as helium, or for increasing production of multiply charged ionic species. In particular, the efficiency of He⁺⁺ production may be enhanced using filaments of the invention. In some embodiments, a mixture of gas may be provided and ionized within the arc chamber. For example, in some embodiments, it may be desirable to provide a helium gas/second gas mixture within the arc chamber to further increase the ionization potential of helium. Suitable helium mixtures and processes have been described, for example, in commonly-owned, co-pending U.S. patent application Ser. No. not yet assigned, entitled “Helium Ion Generation Method and Apparatus”, and filed on Apr. 3, 2002, the disclosure of which is incorporated herein by reference.

[0035] The present invention will be further illustrated by the following examples, which are intended to be illustrative in nature and are not to be considered as limiting the scope of the invention.

EXAMPLE 1

[0036] This example illustrates the production of an ion beam that includes multiply charged helium ions (He⁺⁺) by an ion source using a filament of the invention that has a reduced active surface area as compared to a conventional filament.

[0037] A model EHPi-500, medium current ion implanter from Varian Semiconductor Equipment Associates, Inc. (VSEA), (Gloucester Mass., USA) was modified to include a 250 Volts (V) and 4 Amperes (A) Arc Power Supply and to allow gas pressures approximately 3 times the maximum of about 10 Torr allowed by the commercial machine configuration. The implanter was also modified to allow a source magnet current of 50 A and to permit an extraction current of up to 25 milliamps (mA).

[0038] The filament used in the implanter had the same diameter and distance which the active portion protruded into the chamber as a conventional Bemas-type filament that included a coiled tip portion, typically used in this ion implanter. The total length of the active portion of the filament used was approximately 1.3 inches (3.3 cm) which was less than the 2.0 inches (5.1 cm) for the conventional filament.

[0039] Helium was used as the source gas. The ion source was operated at an arc voltage of about 240 Volts, an arc current of about 4.3 A, a source pressure of approximately 25 Torr and an extraction current of about 15 mA. At these operating conditions, an He⁺⁺ set-up beam current of about 47 μA was measured. This set-up beam current translates into an He⁺⁺ current of about 40 μA at the target wafer.

[0040] Table 1 shows other operating conditions and the measured He⁺ and He⁺⁺ setup beam currents. TABLE 1 Arc Voltage  240 V Arc Current 5.65 A Extraction Voltage   70 kV He Gas Pressure Setup (Beam) Current (Torr) He⁺⁺(μA) He⁺(mA) 7.5 49 6.8 8 63.1 7.7 8.5 70.6 8.2 9 75.6 8.3

EXAMPLE 2

[0041] This example illustrates the increased beam current, and thus ionization efficiency, obtained using a filament having a reduced active surface area in accordance with the invention as compared to a conventional filament.

[0042] The ion implanter described in Example 1 was used. A phosphorous gas source was used. A conventional filament (2.0 inch active portion) was used in one set of trials. A reduced surface area filament (1.3 inch active portion) was used in another set of trials. The conventional filament and the reduced surface area filament had the same diameter and extended the same distance into the chamber.

[0043] Trials with both filaments were conducted at a gas pressure of about 3.85 Torr and an arc current of about 4 Amps. In both sets of trials, the arc voltage was increased from about 20 Volts to 150 Volts. The beam current of P⁺⁺ ions and P⁺⁺⁺ ions were measured at each 10 Volt increment. FIG. 5 compares the beam current of P⁺⁺ ions obtained using the conventional filament versus the beam current the beam current of P⁺⁺ ions obtained using the reduced surface area filament. FIG. 6 compares the beam current of P⁺⁺⁺ ions obtained using the conventional filament versus the beam current of P⁺⁺⁺ ions obtained using the reduced surface area filament. As shown in FIGS. 5 and 6, the beam currents for both P⁺⁺ ions and P⁺⁺⁺ ions obtained using the reduced surface area filament are greater than those obtained using the conventional filament. This is representative of the increased ionization of phosphorous obtained using the reduced surface area filament.

[0044] Additional sets of trials with both filaments were conducted at a gas pressure of about 3.85 Torr and an arc voltage of about 120 Volts. In both sets of trials, the arc current was increased from about 0 Amps to about 4.5 Amps. The beam current of P⁺⁺⁺ ions was measured at each 0.5 Amp increment. FIG. 7 compares the beam current of P⁺⁺⁺ ions obtained using the conventional filament versus the beam current of P⁺⁺⁺ ions obtained using the reduced surface area filament. As shown in FIG. 7, the beam currents for P⁺⁺⁺ ions obtained using the reduced surface area filament are greater than those obtained using the conventional filament. This is representative of the increased ionization of phosphorous obtained using the reduced surface area filament.

[0045] The above description and examples are intended to be illustrative and not exhaustive. The description will suggest many variations and alternatives to one of ordinary skill in this art. All these alternatives and variations are intended to be included within the scope of the attached claims. Those familiar with the art may recognize other equivalents to be specific embodiments described herein which equivalents are also intended to be encompassed by the claims attached hereto. Further, the particular features presented in the independent claims below can be combined with each other in other manners within the scope of the invention such that the invention should be recognized as also specifically directed to other embodiments having any other possible combination of the features of the dependent claims. 

What is claimed is:
 1. An ion source comprising: an arc chamber; and a filament having at least a portion that is located in the arc chamber, the filament including a pair of arm members joined by a non-coiled tip portion, wherein the tip portion defines a V-shape or a U-shape.
 2. The ion source of claim 1, wherein the arm members are generally parallel.
 3. The ion source of claim 1, wherein the arm members define a first plane and the tip portion defines a second plane which intersects the first plane at an acute angle.
 4. The ion source of claim 1, wherein the filament is made of tungsten or tantalum.
 5. The ion source of claim 1, further comprising a source gas entry port formed in the chamber.
 6. The ion source of claim 5, wherein the tip portion is the closest portion of the filament to the source gas entry port.
 7. The ion source of claim 1, wherein an active portion of the filament extends into the arc chamber.
 8. The ion source of claim 1, further comprising a source gas supply.
 9. The ion source of claim 1, wherein the source gas is helium.
 10. The ion source of claim 1, wherein the source gas is a mixture of helium and a second gas.
 11. A method of operating a filament in an ion source comprising: using a first filament that includes an active portion having a first active surface area at first source operating conditions to generate source gas ions at a first efficiency; replacing the first source filament with a second source filament, the second source filament including an active portion having a second active surface area less than the first active surface area; and using the second source filament at the first source operating conditions to generate the source gas ions at a second efficiency greater than the first efficiency.
 12. The method of claim 11, wherein the total length of the active portion of the second source filament is less than the total length of the active portion of first source filament.
 13. The method of claim 12, wherein the active portion of the second source filament is between about 50% and about 80% the total length of the active portion of first source filament.
 14. The method of claim 12, wherein the active portion of the second source filament is between about 60% and about 70% the total length of the active portion of first source filament.
 15. The method of claim 111, wherein the cross-sectional area of the active portion of the second source filament is the same as the cross-sectional area of the active portion of the first source filament.
 16. The method of claim 11, wherein the total length of the active portion of the second source filament is the same as the total length of the active portion of first source filament, and the cross-sectional area of the active portion of the second source filament is less than the cross-sectional area of the active portion of the first source filament.
 17. The method of claim 11, wherein the second source filament includes a pair of arm members joined by a non-coiled tip portion.
 18. The method of claim 17, wherein the non-coiled tip portion defines a V-shape or a U-shape.
 19. The method of claim 17, wherein the arm members define a first plane and the tip portion defines a second plane which intersects the first plane at an acute angle.
 20. The method of claim 11, wherein the first source filament includes a coiled tip portion.
 21. The method of claim 11, wherein the source gas ions generated are multiply charged.
 22. The method of claim 11, wherein the source gas ions generated are helium ions.
 23. A method as in claim 11, wherein the source gas includes helium. 